1. Field of the Invention:
The present invention relates to a spread spectrum communication system and a transmitter-receiver based on CDMA (code division multiple access) and, more particularly, to a spread spectrum communication system and a transmitter-receiver capable of adaptively changing the power level of transmitted signals.
2. Description of the Related Art:
The spread spectrum communication system works in principle as follows: The transmitter of the system modulates (spreads) by pseudo noise (PN) a carrier that carries data. The receiver subjects the received carrier to a PN-coded correlation (reverse spread) process, the PN being generated by an encoder structurally identical to the one used by the transmitter. The PN-code correlation process is followed by base band demodulation that restores the transmitted data. Under this spread spectrum communication scheme, the density of power per unit frequency is low. This means that a minor increase in noise level accompanying a higher traffic, insignificant for other kinds of communication, can lead to a degenerated S/N (signal-to-noise) ratio with the spread spectrum communication system. The raised noise level hampers efforts to communicate using a desired signal under the spread spectrum communication scheme.
One prior art solution to the above problem is to widen the frequency band of spread spectrum signals while lowering the power density per unit frequency. This requires enhancing the clock rate of the transmitter, which means greater power dissipation. In that case, even if traffic is low, the clock rate remains unnecessarily high reflecting the increased power consumption.
FIG. 1 is a block diagram of a modulating portion in a transmitter for use with a conventional direct spread spectrum communication system. In FIG. 1, a carrier generator 201 generates a carrier fc for input to a PSK (phase shift keying) modulator 202. The PSK modulator 202 subjects the carrier fc to bi-phase shift keying modulation using a transmitted signal (binary coded signal) d(t) from an input terminal 203. The PSK-modulated signal from the PSK modulator 202 is supplied to a spread spectrum modulator 204. The spread spectrum modulator 204 is fed with a spread signal p(t) from a PN generator 205 that generates a PN (pseudo noise) code sequence. Using the spread signal p(t), the modulator 204 subjects the PSK-modulated signal to spread spectrum modulation.
FIG. 2A is a view of a typical change in the transmitted signal d(t) used by the modulation portion of FIG. 1. FIG. 2B is a view of a frequency spectrum of the PSK-modulated signal output by the PSK modulator 202 in FIG. 1. In FIG. 2A, Td is the period of the transmitted signal d(t). The frequency band width Bd is given as EQU Bd=1/Td
FIG. 3A is a view of a typical change in the spread signal p(t) from the PN generator 205. FIG. 3B is a view of a frequency spectrum of the spread spectrum signal output by the spread spectrum modulator 204. In FIG. 3A, Tp is the period of the spread signal p(t). As shown in FIGS. 3A and 3B, the period Tp of the spread signal p(t) changes fast over short time with respect to the period Td of the transmitted signal d(t). This causes the spread spectrum modulator 204 to spread the frequency spectrum over a wide band (frequency band width Bp=1/Tp).
FIG. 4 is a block diagram of a demodulating portion of a receiver for use with the direct spread spectrum communication system of FIG. 1. in FIG. 4, the spread spectrum signal received by an antenna or the like, not shown, and admitted through a terminal 211 enters a band-pass filter (BPF) 212. The band-pass filter 212 retains only those components of the signal which constitute the necessary band and discards the rest.
Past the band-pass filter 212, the spread spectrum signal goes into a reverse spread device 213 illustratively made of a multiplier. For its reverse spread operation, the reverse spread device 213 is fed by a PN (pseudo noise) generator 214 with a signal p(t)' identical to the above-mentioned spread signal p(t). In this case, the signal p(t)' from the PN generator 214 is so controlled as to coincide in phase with the spread signal p(t). That is, the relation EQU p(t).multidot.p(t)'=p(t).sup.2 =1
should hold.
The output signal from the reverse spread device 213 goes to a band-pass filter 215 whose center frequency is fc and whose passing band is 2 Bd. The band-pass filter 215 extracts a PSK-modulated signal from the signal received. The PSK-modulated signal is supplied to and demodulated by a PSK demodulator 216. As a result, the original signal d(t) is tapped from an output terminal 217. Spread spectrum communication, as outlined, is a communication method whereby a frequency spectrum is spread over a wide band for communications that ensure security and privacy with high immunity to interference.
To keep the spread spectrum communication system normally operational requires conventionally that the receiving power of the base station remain constant over the communication channels connected to subordinate mobile stations. It is thus necessary to keep constant the transmitting power of each mobile station as it communicates with the base station while moving under varying external conditions. Theoretical calculations put the precision of transmitting power control to within 0.5 dB in the vicinity of the upper limit of the system's circuit capacity. In practice, that kind of precision is difficult to achieve. This has been a major problem with spread spectrum communication systems based on CDMA (code division multiple access).
FIG. 5A is a view of a frequency spectrum of the signal sent from the input terminal 211 to the band-pass filter 212 in FIG. 4. FIG. 5B is a view of a frequency spectrum of the signal sent from the reverse spread device 213 to the band-pass filter 215 in FIG. 4. FIG. 5C is a view of a frequency spectrum of the signal sent from the band-pass filter 215 to the PSK demodulator 216 in FIG. 4. In FIG. 5A, the spread spectrum signal with a band width of 2 Bp mixes with a narrow band interference component. If the power of the signal in FIG. 5A is denoted by PR and that of the interference wave by P.sub.I, the signal-to-interference wave power ratio (S/I).sub.A is given as EQU (S/I).sub.A =Pr/P.sub.I
In FIG. 5B, a reverse relationship of what is given in FIG. 5A holds. That is, the signal passes through the band-pass filter 215 having a band width of 2 Bd. The result is shown in FIG. 5C. In this case, the signal-to-interference wave power ratio (S/I).sub.C is given as ##EQU1## where, G stands for a process gain (G=Bp/Bd). As indicated, subjecting the input signal to spread spectrum modulation improves the signal-to-interference wave power ratio from (S/I).sub.A to (S/I).sub.C, i.e., by the amount of G. Thus the spread spectrum communication scheme enhances the immunity to the adverse effects of interference signal components.
Consider the case where white noise is involved, with no narrow band interference signal component present. In this case, as above, the spectrum patterns of the respective signals in FIG. 4 appear as depicted in FIGS. 6A, 6B and 6C. The signal-to-noise ratio (S/N).sub.A of the signal in FIG. 6A is given as EQU (S/N).sub.A =Pr/(No.multidot.2Bp)
where, No is the power of the white noise signal component. Likewise, the signal-to-noise ratio (S/N).sub.C of the signal in FIG. 6C is given as ##EQU2## The case above thus yields the same result as that of the case where the narrow band interference signal component is involved as depicted in FIG. 5.
In the case of communication within one system whose terminals utilize the same PN code, a given terminal regards the communication done by any other terminal as a noise similar to the white noise. That is, when one terminal transmits its signal at a raised power level, the terminal not only dissipates more power than before but also interferes with the communication of other terminals. Communication carried out under schemes other than the spread spectrum communication constitutes a component approximating the narrow band interference signal component. In any case, higher levels of traffic lead to the increase in the noise indicated by the shaded portions in FIGS. 5C and 6C. This is a significant impediment to the normal execution of communication.
One prior art solution to the above impediment is to enlarge the band width Bp of the spread spectrum signal so as to increase the process gain G. This requires boosting the clock rate for reverse spread operation through multiplication of the spread signal p(t)' in FIG. 4. The solution results in more power dissipation of which the level turns out to be disproportionately high where the process gain G need not be very high.